Priority is claimed to Japanese Patent Application No. 2004-273999, filed Sep. 21, 2004, and Japanese Patent Application No. 2005-090130, filed in Mar. 25, 2005, which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an oxide superconductor with a c-axis oriented parallel to a substrate and an a-axis (or b-axis) oriented perpendicular to the substrate, especially to bismuth based (hereunder “Bi-based”) oxide superconductor thin films, specifically Bi2Sr2Ca2Cu3O10±X (where X is a positive number less than unity, hereunder “Bi-2223”) or Bi2Sr2CuO6±Y (where Y is a positive number less than unity, hereunder “Bi-2201”), in order to obtain a high performance layered Josephson junction using an oxide superconductor especially a Bi-based oxide superconductor, and a method of manufacturing the same.
2. Description of Related Art
A feature of a Josephson device, which uses a superconductor, is its high speed operation and low power consumption. When applied to an integrated circuit, it can perform high speed switching with little electric power. In addition to the high speed switching, the Josephson device shows a small heat production than a high density integrated circuit, in which heat production is a problem common to semiconductor devices. Therefore, it is expected that the Josephson device exhibits a higher speed operation performance compared to a semiconductor.
Conventionally, Nb metal or NbN was used as a superconductor in a Josephson device. However, because the superconductive transition temperature is low, the Josephson device was usually operated at a liquid helium temperature of 4.2K. Compared to this, since an oxide superconductor has a higher superconductive transition temperature, a Josephson device using an oxide superconductor can be operated at around a liquid nitrogen temperature, and thus be it is favorable in the view of resource and energy saving.
A superconductive device that shows the Josephson effect is called a Josephson junction. A Josephson junction, which is suitable for constituting an integrated circuit using superconductive devices, is favorable to be manufactured as a layered junction that has a very thin barrier layer of a normal conductor or an insulator inserted between two superconductive thin films as shown in FIG. 1, as it enables precise dimensional control and the manufacture of many junctions. In practice, a laminated junction is also being used as a Josephson junction in superconductive integrated circuits using Nb metal.
A problem that requires a breakthrough in order to realize manufacturing of a layered Josephson junction using an oxide superconductor, is closely related to the crystal structure of oxide superconductors. Yttrium based (hereunder “Y-based”) oxide superconductors and Bi-based oxide superconductors have more remarkable anisotropy of superconductive characteristics such as coherence length, magnetic flux penetration depth, or critical current density, than for conventional superconductors such as Nb.
The crystals of these superconductors have orthorhombic lattice or tetragonal lattice structure, but the strength of the superconductive coupling in the c-axis direction is weaker than the coupling in the in-plane direction of a surface that is perpendicular to the c-axis. Superconductivity of an oxide superconductor is thought to occur in a CuO plane composed of a copper (Cu) atom and an oxygen (O) atom.
Therefore, the anisotropy of the superconductive coupling derives from the fact that the CuO plane is oriented perpendicular to a c-axis (namely, in the a- or b-axis direction), and not in the c-axis direction. Accordingly, the coherence length (the inter-electronic distance in which a superconducting electron pair is formed) which is closely related in a Josephson junction, is significantly shorter in the c-axis direction than in the a-axis direction. This tendency is more remarkable in a Bi-based superconductor, whose crystal structure has bigger anisotropy than that of a Y-based superconductor, and the coherence length in the c-axis direction is as short as 0.2 nm.
Thus, an oxide superconductor, especially a Bi-based superconductor such as Bi-2223 or Bi-2201 has extremely short coherence length in the c-axis direction. Therefore, in order to manufacture a Josephson junction layered in the c-axis direction using c-axis oriented films it is essential to form a flat and very thin barrier layer. However, in making a barrier layer very thin, a rough surface caused by deposit and so on becomes a problem, which makes it difficult to form a very thin uniform barrier layer, and which causes current leakage between superconductors sandwiching the barrier layer from each side. Therefore this type of Josephson junction has not been obtained yet. Moreover, even if the Josephson junction can be formed, the Josephson critical current density Jc and the Josephson characteristic parameter IcRn are small, and good characteristics may not be obtained.
Accordingly, in order to obtain a high performance layered Josephson junction using a Bi-based oxide superconductor, it is essential to manufacture a junction in the non c-axis direction in which the coherence length is longer than in the c-axis direction. Among these directions, the direction in which the coherence length is the longest is the a-axis (or b-axis) direction. Therefore, in order to obtain a high performance layered Josephson junction using a Bi-based oxide superconductor, it is preferable to manufacture a Bi-based oxide superconductor thin film whose c-axis is oriented parallel to the substrate and whose a-axis (or b-axis) is oriented perpendicular to the substrate.
As one of the methods to realize this, there is known (Japanese Unexamined Patent Application, First Publication No. Hei 5-7027) a method of manufacturing an oxide superconductor film comprising; a step for forming a composition modulated film composed of oxides on a substrate by supplying active oxygen and a part of the metallic components of a Bi-based oxide onto the substrate, and a step for forming an oxide superconductor thin film on the composition modulated film by supplying active oxygen and all of the metallic components of the Bi-based oxide. However, according to this method, the proportion of the c-axis that is parallel to the substrate varies depending on conditions, and it can not be said that a Bi-based oxide superconductor thin film of good quality can be obtained.
Another method is proposed (Japanese Unexamined Patent Application, First Publication No. Hei 9-246611) that a Josephson device using a Bi-based oxide superconductor thin film whose c-axis is oriented parallel to the substrate and whose a-axis (or b-axis) is oriented perpendicular to the substrate, has excellent performance. However, there is no disclosure of any specific process to obtain the good quality Bi-based oxide superconductor thin film whose c-axis is oriented parallel to the substrate and whose a-axis (or b-axis) is oriented perpendicular to the substrate.